1. Field of the Invention
This invention relates to a process for the transformation of vanadium/phosphorus mixed oxide catalyst precursors into active catalysts for the production of maleic anhydride. More particularly, this invention relates to a process for the transformation of catalyst precursors comprising vanadyl hydrogen phosphate, optionally containing a promoter component, into active catalysts comprising vanadyl pyrophosphate, also optionally containing a promoter component. The active catalysts are suitable for the production of maleic anhydride via the partial oxidation of nonaromatic hydrocarbons in the vapor phase with molecular oxygen or a molecular oxygen-containing gas.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It also is a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these varied needs.
2. Description of the Related Art
Numerous catalysts containing vanadium, phosphorus, and oxygen (sometimes referred to as mixed oxides of vanadium and phosphorus), substantially in the form of vanadyl pyrophosphate, optionally containing a promoter component, are disclosed in the prior art as being useful for the conversion of various organic feedstocks to maleic anhydride. In general, such catalysts, wherein the valence of the vanadium is less than +5, usually between about +3.8 and about +4.8, are considered as being particularly well-suited for the production of maleic anhydride from saturated hydrocarbons having at least four carbon atoms in a straight chain (or cyclic structure). In many instances, these catalysts also contain added promoter elements or components which are considered to exist in the catalyst as oxides. Common organic feedstocks include nonaromatic hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof.
Procedures for the preparation of catalysts containing the mixed oxides of vanadium and phosphorus, optionally containing a promoter component, are also disclosed and taught by the prior art. In general, such catalysts are prepared by contacting vanadium-containing compounds, phosphorus-containing compounds, and promoter component-containing compounds (when a promoter element is desired) under conditions sufficient to reduce pentavalent vanadium to the tetravalent state and form the desired catalyst precursor comprising vanadyl hydrogen phosphate, optionally containing a promoter component. The catalyst precursor is thereafter recovered and subjected to a variety of conventional techniques well known to those skilled in the art (commonly and conveniently referred to as calcination or cognate words, such as calcine or calcined) to produce the active catalyst.
U.S. Pat. No. 4,632,916 discloses vanadium/phosphorus oxide catalysts, optionally containing a promoter component of silicon and at least one of indium, antimony, and tantalum, for the vapor-phase oxidation of n-butane to maleic anhydride. Such catalysts are prepared by (a) contacting in an aqueous or organic liquid medium a vanadium species having a valence of substantially +4 with a phosphorus species in an amount sufficient to form a catalyst precursor which will provide a phosphorus/vanadium (P/V) atom ratio in the catalyst of from about 0.9 to about 1.3, and optionally a promoter comprising silicon and at least one of indium, antimony, and tantalum in amounts sufficient to form a catalyst precursor which will provide a silicon/vanadium (Si/V) atom ratio and an (indium+antimony+tantalum)/vanadium [(In+Sb+Ta)/V] atom ratio, respectively, in the catalyst of from about 0.02 to about 3.0 and 0.005 to about 0.2, (b) blending the catalyst precursor with a pore-modifying agent in an amount of from 3% to about 5% by weight and fumed silica having a surface area of at least 150 m.sup.2 /g in an amount of from about 0.05% to about 0.20% by weight, and (c) heating the resultant combination to generate the catalyst.
U.S. Pat. No. 4,632,915 discloses catalysts comprising phosphorus, vanadium, and oxygen, and a promoter component containing each of iron and lithium which are useful for the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, with molecular oxygen or a molecular oxygen-containing gas in the vapor phase to produce maleic anhydride in excellent yields. The catalyst precursors are calcined under conditions involving subjecting the catalyst precursor to elevated temperatures in a variety of atmospheres, including dry air, followed by water-containing air, which, in turn, is followed by hydrocarbon-containing air to yield the active catalyst.
U.S. Pat. No. 4,567,158 relates to a process for the preparation of vanadium/phosphorus mixed oxide oxidation catalysts wherein the catalysts, which exhibit a single pass weight/weight productivity of at least 70 grams of maleic anhydride (from a nonaromatic hydrocarbon) per kilogram of catalyst per hour, are produced from catalyst precursors by subjecting the catalyst precursors to elevated temperatures in an air atmosphere and/or a nitrogen/steam-containing atmosphere for a time sufficient to yield active catalyst.
U.S. Pat. No. 4,562,269 discloses large surface area oxidation catalysts suitable for the conversion of C.sub.4 to C.sub.10 hydrocarbons to maleic anhydride. Such catalysts comprise the oxides of vanadium, phosphorus, and tin in combination with a crystalline silica having a surface area between 100 m.sup.2 /g to 450 m.sup.2 /g wherein the vanadium has an average valence in the range of from about +3.5 to +4.9. The transformation of the catalyst precursor into the active catalyst is accomplished by calcination of the catalyst precursor in an air or oxygen atmosphere at a temperature of from about 204.degree. C. (400.degree. F.) to about 649.degree. C. (1200.degree. F.) for about 0.25 hour to about 6 hours, preferably from about 0.5 hour to about 4 hours.
U.S. Pat. No. 4,333,853 discloses a vanadium/phosphorus mixed oxide catalyst prepared by reducing vanadium substantially in the pentavalent valence state to a tetravalent valence state in the presence of a phosphorus-containing compound and in the absence of a corrosive reducing agent in an organic liquid medium capable of reducing the vanadium to a valence state less than +5, recovering the resultant vanadium/phosphorus mixed oxide catalyst precursor, drying such precursor, and calcining the precursor, preferably in the presence of an oxygen-containing gas, to obtain the active catalyst. Such catalysts reportedly are effective in the oxidation of C.sub.4 hydrocarbons such as n-butane, 1- and 2-butenes, 1,3-butadiene, or mixtures thereof to produce maleic anhydride with selectivities ranging from 58.7% to 68.1% and yields (mol %) ranging from 51.4% to 59.5%.
U.S. Pat. No. 4,315,864 relates to a process for the production of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a vanadium/phosphorus mixed oxide catalyst. The catalyst is prepared by reducing a pentavalent vanadium-containing compound in an olefinic, oxygenated organic liquid medium to a +4 valence in the absence of a corrosive reducing agent, recovering resultant catalyst precursor, drying the catalyst precursor, and calcining the precursor, preferably in the presence of an oxygen-containing gas, to obtain the active catalyst.
U.S. Pat. No. 4,312,787 describes a catalyst which comprises an inert support and a catalytically active mixed oxide material coating of vanadium and phosphorus or of vanadium, phosphorus, and uranium on the outer surface of the support in an amount greater than 50% to about 80% by weight of the combined support and oxide material. Catalysts within the scope of the claims of the patent were reported to produce maleic anhydride from n-butane in yields ranging from 53% to 62.5%, with selectivities ranging from 57.4% to 67.9%.
In U.S. Pat. No. 4,251,390, a zinc-promoted vanadium-phosphorus-oxygen catalyst is disclosed and claimed. The catalyst is prepared by reducing pentavalent vanadium in a substantially anhydrous organic medium to a lower valent state and digesting the reduced vanadium in the presence of a zinc promoter compound. The resultant catalyst (catalyst precursor) is activated by a rapid conditioning procedure involving heating the catalyst to a temperature of 380.degree. C. in a stream of air at a temperature increase of 3.degree. C. per minute, maintaining these conditions for 2 hours, and thereafter increasing the temperature to 480.degree. C. at 4.degree. C. per minute in the presence of a butane-in-air mixture, or alternatively by a standard conditioning procedure involving bringing the catalyst to operating temperatures for the oxidation of n-butane to maleic anhydride at a rate of 5.degree. C. to 10.degree. C. per hour in the presence of a butane-in-air mixture.
In U.S. Pat. No. 4,187,235, a process is described for preparing maleic anhydride from n-butane in the presence of a vanadium-phosphorus-oxygen high surface area catalyst, that is, 10 to 100 square meters per gram (m.sup.2 /g), as determined by the BET method. The catalyst is prepared by reducing pentavalent vanadium to a valence between +4.0 and +4.6 with a substantially anhydrous primary or secondary alcohol and contacting the reduced vanadium with phosphoric acid, followed by recovering and calcining the resultant vanadium(IV) phosphate catalyst precursor compound by heating the catalyst precursor to a temperature of 380.degree. C. in a stream of air at a temperature increase of 3.degree. C. per minute, maintaining these conditions for two hours, and thereafter increasing the temperature to 480.degree. C. at 4.degree. C. per minute in the presence of a butane-in-air mixture.
U.S. Pat. 4,018,709 discloses a process for the vapor phase oxidation of normal C.sub.4 hydrocarbons using catalysts containing vanadium, phosphorus, uranium, or tungsten or a mixture of elements from zinc, chromium, uranium, tungsten, cadmium, nickel, boron, and silicon. In a preferred embodiment, the catalyst also contains an alkali metal or an alkaline earth metal, especially lithium, sodium, magnesium, or barium as active components. Typically, the active catalysts are prepared by refluxing a reaction mixture of suitable source materials in concentrated (37%) hydrochloric acid to form a catalyst precursor. The resultant catalyst precursor is recovered, dried, and calcined in air at elevated temperatures of from about 300.degree. C. to about 350.degree. C.
In U.S. Pat. No. 3,980,585, a process is disclosed for the preparation of maleic anhydride from normal C.sub.4 hydrocarbons in the presence of a catalyst containing vanadium, phosphorus, copper, oxygen, tellurium, or a mixture of tellurium and hafnium or uranium or a catalyst containing vanadium, phosphorus, copper, and at least one element selected from the group of tellurium, zirconium, nickel, cerium, tungsten, palladium, silver, manganese, chromium, zinc, molybdenum, rhenium, samarium, lanthanum, hafnium, tantalum, thorium, cobalt, uranium, and tin, optionally (and preferably) with an element from Groups IA (alkali metals) or IIA (alkaline earth metals). The prepared catalyst precursor is converted into the active catalyst by calcining the catalyst precursor in air at elevated temperatures of from about 300.degree. C. to about 350.degree. C.
U.S. Pat. No. 3,888,886 discloses a process for the oxidation of n-butane at a temperature from about 300.degree. C. to about 600.degree. C. with a vanadium/phosphorus/oxygen catalyst having a phosphorus/vanadium atom ratio of from about 0.5 to about 2, promoted or modified with chromium, iron, hafnium, zirconium, lanthanum, and cerium, the promoter metal/vanadium atom ratio being between about 0.0025 and about 1. The catalysts are prepared by refluxing a reaction mixture of vanadium oxide, phosphoric acid, a hydrogen halide (usually hydrochloric acid), and a specified promoter metal-containing compound to yield the corresponding catalyst precursor. The resultant catalyst precursor is recovered, dried, formed into structures--spheres, for example--and calcined in the presence of a butane-in-air mixture at about 490.degree. C. to produce the active catalyst.
U.S. Pat. No. 3,864,280 discloses vanadium/phosphorus mixed oxide catalyst having an intrinsic surface area of from about 7 to about 50 m.sup.2 /g. The catalysts are prepared by precipitation of a vanadium/phosphorus/oxygen complex from an essentially organic solvent medium in the absence of gross amounts of water. The resultant crystalline precipitate is activated by heating in air, followed by a 1.5 mol % butane-in-air mixture, both at elevated temperatures.
U.S. Pat. No. 3,862,146 discloses a process for the oxidation of n-butane to maleic anhydride in the presence of a vanadium-phosphorus-oxygen catalyst complex, promoted or activated with zinc, bismuth, copper, or lithium activator, obtained by calcination of a catalyst precursor in air at elevated temperatures. The phosphorus/vanadium and activator/vanadium atom ratios are from about 0.5 to about 5 and from about 0.05 to about 0.5, respectively.
U.S Pat. No. 3,856,824 discloses a process for the production of maleic anhydride by oxidation of saturated aliphatic hydrocarbons in the presence of a catalyst comprising vanadium, phosphorus, iron, oxygen, and added modifier comprising chromium combined with at least one element selected from the group consisting of nickel, boron, silver, cadmium, and barium. The active catalysts are prepared by refluxing an aqueous slurry of suitable source materials acid to form a catalyst precursor. The resultant catalyst precursor is recovered, dried, and calcined in air, oxygen, or an inert gas, preferably air, at elevated temperatures of from about 400.degree. C. to about 600.degree. C.
European Patent Application No. 98,039 discloses a process for the preparation of vanadium-phosphorus mixed oxide catalysts, optionally containing an added promoter element selected from the group consisting of Group IA (alkali metals), Group IIA (alkaline earth metals), titanium, chromium, tungsten, niobium, tantalum, manganese, thorium, uranium, cobalt, molybdenum, iron, zinc, hafnium, zirconium, nickel, copper, arsenic, antimony, tellurium, bismuth, tin, germanium, cadmium, and lanthanides, and mixtures thereof. The catalysts, which exhibit a phosphorus/vanadium atom ratio of from about 0.8 to about 1.3 and a promoter/vanadium atom ratio from about 0.01 to about 0.5, are prepared in an organic liquid reaction medium capable of reducing the vanadium to a valence state of approximately +4 to form a nonsolubilized catalyst precursor, contacting the nonsolubilized catalyst precursor-containing organic liquid with water to form a two-phase system having an upper organic liquid phase and a lower nonsolubilized catalyst precursor-containing aqueous phase, drying the catalyst precursor, and calcining the precursor at a temperature of from 300.degree. C. to 500.degree. C. in the presence of air, hydrocarbon, an inert gas, or a mixture of steam and air to obtain the active catalyst. The catalysts so obtained reportedly are useful in the production of maleic anhydride from normal C.sub.4 hydrocarbons.
The transformation of the catalyst precursor into the active catalyst, as indicated by the cited references, may be accomplished by calcination under a variety of conditions. And although the prior art processes generally are effective to provide the desired active catalyst (from the catalyst precursor), which, in turn, generally are successful in producing the desired maleic anhydride product, the commercial utility of a catalyst system and a catalytic process is highly dependent upon the cost of the catalyst employed, the conversion of the reactants, and the yield of the desired product(s), or stated differently, the actual productivity of the catalyst system. In many instances, a reduction in the cost of a catalyst system employed in a given catalytic process on the order of a few cents per kilogram or pound, or a small percent increase in the yield of the desired product, relative to the amount of catalyst required, represents a tremendous economic advantage in a commercial operation. Accordingly, research efforts are continually being made to define new or improved catalyst systems and methods and processes of making new and old catalyst systems to reduce the cost and/or upgrade the activity, selectivity, and/or productivity of such catalyst systems in such catalytic processes. The discovery of the process of the instant invention, therefore, is believed to be a decided advance in the catalyst art.